The present invention relates to a method of manufacturing leaded, free-cutting steel with a high yield of lead addition by a continuous casting process.
Conventionally, in manufacturing leaded, free-cutting steel, lead or lead alloy has been added to molten steel contained in a ladle or to molten steel being poured into an ingot mold. As specific methods for adding lead or lead alloy to the molten steel, there have heretofore been practiced a method involving a direct addition of lead or lead alloy to molten steel being tapped from a furnace into a ladle, a method comprising blowing an inert gas from the bottom of a ladle into molten steel in the ladle so as to partially remove the slag on the surface of the molten steel and expose the surface of the molten steel, and adding lead or its alloy to the exposed molten-steel, and, if desired, thereafter applying mechanical stirring force to the Pb-containing molten steel; and further, as a way of adding the lead component in the pouring step, there has been proposed a method involving projecting shots of lead or lead alloy into the molten steel stream flowing from the bottom of the ladle, using a projecting machine. However, the above conventional methods of adding lead can not successfully achieve leaded, free-cutting steels in which the lead is uniformly distributed. For example, when lead is directly added to molten steel in a ladle, the molten steel is required to be fully stirred in order to achieve an uniform distribution of lead therein. If the molten steel is not sufficiently stirred, lead is apt to sink to the base part of the steel because lead has a heavy specific gravity compared with iron. As a result, distribution of lead in the product is not uniform. Further, in the above conventional methods, an unfavorable sedimentation of lead occurs during the solidification of the lead-containing molten steel in an ingot mold, decreasing the uniformity of lead distribution. Particularly, when free-cutting steel containing a relatively large amount of lead is desired, much lead should be added to the molten steel, taking the vaporization of lead into consideration, because lead is apt to vaporize during the production process. In this case, the uniformity of the lead distribution in the steel is greatly decreased and the yield of lead addition will be reduced. Further, lead is very toxic, and, thus, not only are special considerations and expensive, complicated facilities inevitably required to treat fire bricks of a ladle or runner bricks used for producing ingots which have been polluted by lead, but also lead vapor generated in the production process should be collected to avoid diffusion in the atmosphere. Thus, expensive collecting equipment is needed to collect toxic gases or fumes not merely in some limited areas but rather in the entire factory. Further, if a conventional continuous casting process which has not been applied to the manufacturing of leaded, free-cutting steel is applied to the manufacturing of leaded, free-cutting steel, without any modification, bricks from a tundish will be polluted by lead and the above problems will be presented.